A transport/storage container for spent nuclear-fuel elements typically has a vessel having a side wall with an inner surface defining an interior extending along an axis and a multilevel basket extending substantially a full axial length of the interior and forming a plurality of axial full-length rectangular-section wells adapted to receive the spent fuel elements. The lower end of the interior is closed by a permanent floor and the upper end by a massive but removable cover.
It is essential to maintain the rods held in such a container in a subcritical state. Thus their neutron emissions must be controlled. This is normally accomplished as described in U.S. Pat. No. 5,032,348 by forming each level of the basket of a plurality of fitted-together plates. Each such set of plates forms a plurality of openings that together form the wells that receive the spent fuel rods. All the plates fit together at slot joints at the corners of all the wells. The neutron-absorbing plates are normally of a relative poor thermal capacity so it is necessary to alternate layers made of plates of neutron-absorbing material with layers made of plates of a more thermally conductive material so that the heat generated in such a container can be conducted to the side walls.
Such a system therefore trades dissipating heat off against suppressing neutron emissions. One function can only be made better by making the other worse. In addition fitting together the numerous plates making up each level is an onerous task, involving meticulously fitting together long plates simultaneously at multiple joints so that the container is expensive to manufacture of a large number of pieces of different sizes.
In another system borated-steel plates form the baskets. Plates of this material must be welded or screwed together, as the slot joint described above is not usable because of the brittleness of borated steel. Thus assembly of the basket becomes extremely expensive since it is too complex to automate, even with welding which is the preferred and cheaper solution. Furthermore the container is extremely heavy when its basket is made of steel.
It is further known for some of the wells in a borated-steel basket to be water filled, acting as neutron traps. Such use of water makes the container quite large and of course also increases its weight, while making the gaps necessary for flow of the water increases the cost of the plates. Furthermore using some of the wells for water only reduces the number of wells usable for fuel rods, reducing the capacity of the container. The basket furthermore is not particularly strong and has difficulty meeting requirements in this regard, as the basket must keep the fuel rods apart even if dropped or otherwise subjected to some serious axial and/or radial stress. Finally, such a system is difficult to decontaminate.